The present invention relates generally to an apparatus for controlling the movement and positioning of a translationally movable system and more particularly to a control apparatus for the read-write heads of a data processing memory system.
In the present data processing systems, magnetic disc memories are being used with increased frequency. Storage capacity and the comparatively short access times to the data contained on the discs are the primary reasons for the usage increase.
All disc memories have or are characterised by a "mean access time." Mean access time is the time required by the heads, which read from or write upon the magnetic disc, to move from an outside track on the disc and gain access to a data item contained on the central track. The time is measured from the moment the heads receive an order from the processing system to seek the data item. The central track of the disc has a serial number equal to the mean of the outside track serial numbers. For example, if there are two hundred recording tracks numbered from 0 to 200, then the central track is track 100.
In practice, the mean access time is determined primarily by two factors. One is the average time taken by the disc to complete one revolution. The second is the average time taken by the magnetic heads to move from the outside track to the central track.
Much development work is presently directed towards reducing mean access time. It is, or course, possible to reduce the average revolution time of a disc by increasing the speed of rotation. However, the mechanical stresses exerted on the disc severely limit this approach. Thus, the perference is to reduce the average time necessary to move the magnetic heads by improving the head control apparatus.
It is to be understood that the primary concern is to reduce the time required to move the magnetic heads from a starting track to a destination track, regardless of starting or destination track. Thus, reduction of mean access time, i.e., where the starting and destination tracks are an outside track and the central track, respectively, is a special case and only illustrative of the general objective.
A majority of head control apparatus includes a "voice coil" electrodynamic motor. The motor has a coil which moves linearly within a permanent magnet, defining a cylindrical core. The coil is mechanically linked to a carriage which bears the magnetic heads. Preferably, the carriage moves along two parallel rails. The coil, carriage and magnetic heads, in combination, define a movable head assembly. The movable head assembly also includes a transducer, which measures the assembly speed at any instant and produces a corresponding speed signal.
With the presently known control apparatus, a reduction in transit time is achieved by two-stage movement. The first stage is an acceleration stage, during which a positive current is applied to the coil of the motor. The speed of the movable head assembly or carriage, as a function of time, is substantially linear and increasing. Speed as a function of the position occupied by the movable assembly is parabolic and also increasing.
During the second stage of movement, or the deceleration stage, an opposite current is applied to the motor. Speed as a function of time and position, in this stage, is linear-decreasing and parabolic-decreasing, respectively. At the end of the second stage, the speed of the movable head assembly is sufficiently low to stop the heads above the selected disc track.
During the first stage, the control apparatus operates under conditions of freedom or, more particularly, is uncontrolled. Conversely, second stage operation is controlled and the speed of the movable assembly is governed to closely approximate speed under conditions of freedom.
Such a control apparatus is a "bang-bang" type. The first stage of the movement is termed the first bang, while the second phase is the second bang.
The known bang-bang control apparatus include means for applying constant current to the motor coil during the first stage of movement. During the second state, the amount of current is determined by comparing the actual speed of the carriage assembly with a theoretical reference speed. The theoretical reference speed is the speed at which the head assembly would be moving under free conditions.
More particularly, the known and existing control apparatus include an electric motor, a speed transducer or first generator for producing a signal representing the actual speed of the movable head assembly, a second generator for producing a signal representing a theoretical reference speed, a comparator for generating a signal representing the difference or error between the actual and theoretical reference speeds, and power means for amplifying the error signal from the comparator. The motor is driven by the amplified error signal.
These control apparatus are subject to a large number of disruptive phenomena of physical or mechanical origin, such as variations in the supply voltage to the motor, changes in the electrical and magnetic characteristics of the motor with time, changes in the mechanical characteristics of the carriage bearing the heads, sensitivity of the speed transducer, etc. The disruptive phenomena upset and vary the movement of the movable assembly.
To substantially avoid variance in movement, the theoretical reference speeds for the movable head assembly are chosen with respect to the least favorable operating conditions. However, under these conditions, it is virtually impossible to achieve an adequate reduction in actual transit times or mean access time.